Direct reduction of iron oxide containing fine-grained materials, such as iron ores, iron ore concentrates or iron ore-containing intermediate products, by means of reducing gases in a fluidized bed, yields a sponge iron product (DRI) which has pyrophoric properties and for this reason must be aftertreated.
Processes for a direct reduction of such materials to produce sponge iron and for a carburization to form Fe.sub.3 C have also been proposed. The product which contains Fe.sub.3 C is not pyrophoric and may be stored and transported without an aftertreatment. Besides, it contains enough carbon from the reduction of the residual iron oxide and for generation of heat as the Fe.sub.3 C-containing product is melted.
Published German Application 27 00 427 and U.S. Pat. No. 32,247 disclose for the production of Fe.sub.3 C a process in which fine-grained iron oxide is reacted to Fe.sub.3 C in a conventional fluidized bed. A hot reducing gas is introduced as a fluidizing gas into the fluidized bed. The fluidizing gas contains H.sub.2, CO, CH.sub.4, CO.sub.2, N.sub.2, and H.sub.2 O.
The ratio between H.sub.2 and the carbonaceous components is preferably so adjusted that the hydrogen effects the reduction to metallic iron and the carbon effects the carburization to Fe.sub.3 C because in that case only water is formed as a gaseous reaction product and can be removed from the exhaust gas by condensation. The ratio of H.sub.2 to the resulting water is kept between 2.5:1 and 8:1 and the ratios of CO to CO.sub.2 and of H.sub.2 to H.sub.2 O are selected to substantially maintain an equilibrium with CH.sub.4. The ratio of CO to CO.sub.2 lies between 1:1 and 4:1.
Analyses of the exhaust gas from the fluidized bed reveal 58.3 to 77% H.sub.2, 0.5% N.sub.2, 5.2 to 7.9% CH.sub.4, 8.9 to 21.4% CO, 2.0 to 6.8 CO.sub.2, balance water vapor, and the Fe.sub.3 C product contains 4.35 to 8.96% C. The temperature in the fluidized bed is required to lie between 482.degree. and 704.degree. and the range from 549.degree. and 632.degree. C. is particularly preferred.
The exhaust gas is cooled in an indirect heat exchanger and is then cooled with water in a scrubber below the dewpoint temperature of the water vapor, whereby the water vapor content is substantially condensed and dust is scrubbed off at the same time.
The purified exhaust gas is preheated in the heat exchanger and is then heated further in a reheater and is replenished by an addition of reducing gases and then recycled as a fluidizing gas to the fluidized bed reactor.
The Fe.sub.3 C product is directly charged to a furnace whose off gas is used to replenish the recycled gas. In a conventional fluidized bed, fresh material becomes very quickly mixed in the bed so that the material which is discharged always contains a part of the unreacted oxide material. Besides, the pressure drop from the wind box to the fluidized bed may vary so that the gas distribution is not uniform.
U.S. Pat. No. 5,118,479 discloses a process by which the above-described disadvantages of the normal conventional fluidized bed are allegedly avoided. In that process, a plurality of spaced apart metal plates which are at right angles and parallel to each other are arranged in the reactor that contains the conventional fluidized bed. Alternate metal plates are adjoined at one end to opposite walls of the reactor and at the other end of each of said plates defines a gap with the wall of the reactor. As a result the charged fresh material flows along a labyrinth path from the inlet to the outlet. The fluidizing gas preferably contains (in mole percent) up to 20%, preferably 50 to 10%, CO, up to 20%, preferably 2 to 8%, CO.sub.2, up to 80%, preferably 35 to 50%, CH.sub.4, up to 80%, preferably 35 to 50%, H.sub.2, 0 to 15%, preferably 0 to 10%, N.sub.2, up to 5%, preferably 1 to 2%, water vapor. The reaction is effected under a pressure of 1 to 3.1 bars, preferably 1 to 2.1 bars. The fluidizing gas is introduced at a temperature from 500.degree. to 750.degree. C., preferably 600.degree. to 700.degree. C. The temperature in the gas space over the fluidized bed is 500.degree. to 600.degree. C. In a conventional fluidized bed containing the above-described internal inserts the reaction conditions are poor because the velocities are low. For a high throughput rate, a reactor which is large in diameter is required so that it is even more difficult to achieve a uniform distribution of gas.
In WO 92/02646 it is proposed to preheat in an oxidizing atmosphere at least a part of the fresh material before it is charged into the conventional fluidized bed. Preheating is effected to 500.degree. to 900.degree. C. and serves to oxidize Fe.sub.3 O.sub.4 at least in part to Fe.sub.2 O.sub.3, to remove sulfide sulfur and water and to preheat the charge. The preheated material is reduced and carburized in a conventional fluidized bed, in which the material is conducted along a labyrinth path as described hereinbefore.